This invention relates to fully-filled telecommunication cables and compositions for use as a filling medium in them. These cables comprise a multiplicity of conductors each with individual insulation, grouped in any appropriate way and enclosed in a sheath, the interstices between the insulated conductors, and between them and the sheath, being filled with a waterproof filling medium. The function of the filling medium is to provide a dielectric medium, the properties of which remain at the required level even when water gains access to the interstices. Furthermore the filling medium must inhibit the passage of water along the interstices in the event that the sheath or a joint enclosure is damaged when the cable is exposed to wet conditions, for instance while buried in the ground.
The insulation of the individual conductors of such cables is almost always of polyethylene or polypropylene (including certain of their semicrystalline thermoplastic copolymers of low permittivity), and in many cases this material is used in cellular form. To be effective the filling medium must be hydrophobic and highly deformable throughout the temperature range experienced by telecommunication cables in service (which can extend from -40.degree. C. to +80.degree. C.). It must also contact the insulation in such a way as to prevent passage of water between itself and the insulation. There are very few non-volatile materials that satisfy these requirements as well as the other important requirements of low permittivity and low dielectric loss, but for cables operating at temperatures up to about 50.degree. C. hydrocarbon oils gelled with waxes (e.g. petroleum jellies) or hydrocarbon oils gelled with other suitable gelling agents have been found to be suitable.
It is however desirable to use fully-filled cables in some circumstances at temperatures up to about 80.degree. C.--for example when they are associated with and run alongside large power cables--and in this case two difficulties arise. First, petroleum jelly shows a tendency to fill cells in the insulation to an extent that may be appreciable in a few months. This can result in partial loss of the medium from interstitial spaces in the cable leading to the development of longitudinal channels and the consequent impairment of the waterproofness of the cable; in addition, the displacement of air (permittivity approximately 1.0) in the cellular insulation by the filling medium (permittivity typically 2.3) will lead to changes in cable capacitance and hence transmission characteristics of the cable. Second, the viscosity of the medium decreases to the point at which it may flow along the interstices under the hydrostatic pressures that may occur in an installed cable, allowing the water to flow after it.